In synchronous AM detectors, the detected output is a function of the phase as well as the amplitude of the modulated carrier. Noncorrelated interfering signals, such as impulse noise, can be of anti-phase with respect to the reference, which causes an output polarity opposite to that of the desired envelope. In television, the normal detection polarity is negative; that is, the largest carrier level produces the most negative output (corresponding to black on the screen). Impulse noise undesirably causes large positive output spikes, which result in brilliant white spots on the screen. Therefore, additional circuitry, commonly called a white-spot inverter, must be added to invert these spots to less visible black. The inverter must be fast, but must not interfere with normal detection.
A further problem with monolithic synchronous detectors is that they necessarily must use NPN multipliers to operate at TV and radio intermediate frequencies. The output of such detectors is a current flowing out of the multiplier, which must be level-shifted and amplified to produce a usable output. U.S. Pat. No. 3,908,171 teaches one way in which zener diodes may be used to accomplish level-shifting, but does not address the requirement of white-spot inversion.